At one point I was considering using s_files, the list of files associated with
a single superblock, to find open state in need of reclaiming after a server
reboot.  So I wanted to understand how the s_files list is used, and how to
traverse it safely.

A file is added to the list of open files *before* the file->open method is
called; see dentry_open:
        file_move(f, &inode->i_sb->s_files);

        if (f->f_op && f->f_op->open) {
                error = f->f_op->open(inode,f);

where file_move installs file in the file list, from file_table.c:
	void file_move(struct file *file, struct list_head *list)
	{
		if (!list)
			return;
		file_list_lock();
		list_move(&file->f_list, list);
		file_list_unlock();
	}

and file_list_{un}lock is just spin_{un}lock(&files_lock).


where sequence of calls is something like:
	sys_open:
		filp_open:
			open_namei:
				path_lookup's
				vfs_create if necessary
			dentry_open

Examples of other places s_files is used:
	add_dquot_ref (in quota code):
		file_list_lock()
		for each file in sb->s_files
			if need to do something with this file, 
				file_list_unlock(), do it,
				then start everything over.
		file_list_unlock()
	fs_may_remount_ro:
		file_list_lock()
		for each file in sb->s_files
			if file is to be deleted, of if file's open for
			write, unlock and return 0
		file_list_unlock(), return true.

Similarly, for per-filesystem and global lists of superblocks, how is iterating
over them handled?

One example is sync_inodes, which does a while(get_super_to_sync()), where
	get_super_to_sync:
		spin_lock(&sb_lock)
		loop over superblock list:
			if we find one with s_syncing false,
				set s_syncing = 1
				sb->s_count++;
				spin_unlock(&sb_lock)
				down_read(&sb->s_umount)
				if !sb->s_root:
					drop_super(sb)
					goto restart
				return sb;
		spin_unlock(&sb_lock);
		return NULL;

so basically this is the same as the add_dquot_ref above; we have a flag to
mark sb's we've already seen, and each time through the list we grab the
spinlock again and start over from the beginning.